A great deal of research has been conducted in recent years on thin-film EL elements as small or large/lightweight flat display panels. Monochrome thin-film EL displays employing fluorescent thin-films composed of yellowish-orange emitting manganese-added zinc sulfide are already implemented, in forms with a bilayer insulating structure using thin-film insulating layers 4A, 4B as shown in FIG. 2. FIG. 2 shows a lower electrode 3A with a prescribed pattern formed on a base 2 made of glass, with a dielectric thin-film formed as a lower insulating layer 4A on the lower electrode 3A. Also, a luminescent layer 5 made of a fluorescent thin-film and an upper insulating layer 4B are formed in that order on the lower insulating layer 4A, while an upper electrode 3B is formed with a prescribed pattern on the upper insulating layer 4B, forming a matrix with the lower electrode 3A. The fluorescent thin-film is usually subjected to annealing treatment below the distortion point of the glass base 2, for enhanced brightness.
Recently there has been proposed a structure wherein the base 2 is composed of ceramic, and a thick-film dielectric layer is used as the lower insulating layer 4. There have also been proposed element structures wherein a high dielectric BaTiO3 thin-sheet is used as the base and an electrode is formed on the back side of the base, where the thin-sheet is used as both the insulating layer and base. Since ceramics such as alumina or BaTiO3 are employed as the bases in such structures, it is possible to carry out high temperature annealing of the fluorescent thin-film, permitting a higher degree of brightness. In addition, because such structures employ thick-film or thin-sheet dielectric layers as insulating layers, they typically can yield elements with greater resistance to dielectric breakdown and higher reliability compared to EL elements using thin-films as insulating layers. It is not necessary in all cases for the structure to be one in which the fluorescent thin-film is sandwiched by insulating layers as in bilayer insulated structures. That is, the insulating layer may be provided as a thick-film or thin-sheet dielectric layer on only one side of the fluorescent thin-film.
Color display is indispensable for applications to personal computer, TV and other display purposes. Thin-film EL displays employing sulfide fluorescent thin-films have high reliability and environmental resistance, but at the current time EL fluorescent bodies which emit the 3 primary colors of red, green and blue exhibit insufficient characteristics and are still unsuitable for color display. Potential blue-emitting fluorescent bodies include those employing SrS as the matrix material and Ce as the luminescent center material (hereinafter indicated as “SrS:Ce”), or SrGa2S4:Ce, ZnS:Tm or the like. Potential red-emitting fluorescent bodies include ZnS:Sm, CaS:Eu and the like. Potential green-emitting fluorescent bodies include ZnS:Tb, CaS:Ce and the like. Research on these materials is currently underway.
Thiogallate-type or thioaluminate-type blue fluorescent bodies such as SrGa2S4:Ce, CaGa2S4:Ce, BaAl2S4:Eu and the like are disclosed in Japanese Patent Application Laid-Open No. HEI 7-122364, Japanese Patent Application Laid-Open No. HEI 8-134440, Shingaku Giho EID98-113, pp. 19-24 and Jpn. J. Appl. Phys. Vol. 38 (1999), pp. L1291-1292, as fluorescent bodies intended to solve the aforementioned problems. These thiogallate-type fluorescent bodies are satisfactory from the standpoint of color purity. However, because the fluorescent bodies comprise multiple elements it is difficult to form thin-films with uniform compositions. The difficulty of controlling the composition leads to reduced crystallinity, generation of defects due to remove of sulfur, and inclusion of impurities. It is therefore assumed that the desired brightness cannot be achieved since high-quality thin-films are unobtainable. Both thiogallate-type fluorescent bodies and thioaluminate-type fluorescent bodies have relatively high film forming process temperatures of 750-900° C., particularly for the annealing temperatures after film formation. This has been the cause of numerous problems, such as a requirement for very high heat resistance of the base and restrictions on base materials, a tendency toward diffusion of elements from the base or adjacent layers (insulating layer, etc.) into the fluorescent thin-film, a tendency for lower flatness between layers, a tendency toward interlayer peeling during high-temperature annealing, a tendency toward collapse of picture elements due to surface diffusion during high-temperature annealing, and increased costs due to the need for heating strategies in annealing apparatuses for high-temperature annealing.
In addition, in order to realize full-color EL panels it is necessary to use fluorescent materials from which blue, green and red fluorescent bodies can be produced in a stable and economical manner. However, because the process temperatures of the aforementioned fluorescent thin-films differ according to their materials, the conditions for forming each of the fluorescent thin-films for obtaining the prescribed luminescent characteristics differ for full color panels wherein the three colors RGB must be situated in the panel; such panels have therefore been difficult to produce. Fluorescent bodies using thioaluminate-type materials and thiogallate-type materials in particular have high process temperatures as mentioned above, and therefore lowering of the process temperature is a desired aim. In other words, it is desirable to form and anneal high brightness-emitting red, blue and green fluorescent thin-film materials simultaneously and at low temperature.
Incidentally, one method of producing high-purity, high-quality sulfide fluorescent thin-films is a method of forming a sulfide fluorescent thin-film by sputtering using a sulfide sintered body as the target.
When a sulfide fluorescent thin-film is produced by sputtering, the compositional ratio of sulfur in the thin-film is lower than in the target, and a thin-film with insufficient sulfur is formed. Thus, methods for avoiding insufficient sulfur have been attempted, such as a method of compensating for insufficient sulfur by introducing H2S gas during the sputtering as described in SID 94 DIGEST page 129, or a method of annealing in a sulfur atmosphere after thin-film formation.
Nevertheless, the methods described above for avoiding insufficient sulfur are highly dependent on the sulfur supply conditions, and the conditions which yield sulfide thin-films with the desired composition and high crystallinity are limited. In addition, since H2S and sulfur are harmful gases, the processes for mass production of such thin-films require noxious gas removal equipment and safety measures, creating an excessive cost burden.
Sputtering targets used for formation of sulfide fluorescent thin-films by sputtering are described, for example, in Japanese Patent Application Laid-Open No. 2001-118677. This publication proposes a sputtering target for an inorganic EL fluorescent thin-film, comprising a matrix material composed mainly of a Group II-sulfur compound, a Group II-Group III-sulfur compound or a rare earth sulfide, and one or more from among magnesium sulfide (MgS), calcium sulfide (CaS) and zinc sulfide (ZnS) at 3-100 mole percent in terms of MgS, CaS and ZnS. As a specific example in this document there is disclosed creation of a sintering target using SrS as the matrix material and adding Zn thereto. The document also states that the use of a sputtering target with ZnS added eliminates sulfur insufficiency in the formed thin-film.